Adhering polar (charged) surfaces to non-polar (neutral) surfaces tends to be problematic, often involving expensive and specialized equipment and inconvenient application techniques. For example, in the automotive industry, it is necessary to bond EPDM-based weatherstripping (non-polar) to an automobile's painted surfaces (polar). This has been performed traditionally using a heat-activated dual-sided tape. To apply this tape, however, the weatherstripping manufacturers must employ heat laminating machinery which uses torches to blast hot air at the nip point between the heat activated adhesive and the EPDM-based weatherstripping. Such heat-activated adhesive not only involves specialized and expensive equipment, but also tends to be inconvenient and, at times, dangerous.
Rather than heat-activated adhesives, it is highly preferable from a convenience and expense standpoint to use pressure-sensitive adhesives. Such adhesives, however, tend to be inadequate in bonding polar surfaces to non-polar surfaces. The difficulty arises because pressure-sensitive adhesives are specific to polar and non-polar applications. That is, they tend to bond strongly with only their respective surfaces. Any bonding between non-polar adhesives and polar surfaces and polar adhesives and non-polar surfaces is usually a consequence of the adhesive's tack rather than chemical bonding. Therefore, there is a need to combine the bonding properties of polar and non-polar adhesives to adequately adhere a polar surface to a non-polar surface.
One approach to bonding polar and non-polar surfaces is to use a blend of non-polar and polar adhesives. Unfortunately, members from one class of adhesives are generally not miscible with, and do not adhere to, members of the other class. More specifically, the interfacial regions between the domains of each component are weak and result in adhesive splitting when subjected to stress. As used herein, the term "adhesive splitting" refers generally to the physical separation of one adhesive component from another adhesive component. In addition to being susceptible to adhesive splitting, a blend necessarily dilutes the concentration of a particular adhesive in the blend. Reducing an adhesive's concentration reduces its adhesive strength. For this reason, it is preferable to have distinct, or "dedicated," layers of concentrated adhesive for bonding to their respective polar or non-polar surfaces. However, the surface area available for interfacial bonding between polar/non-polar adhesive layers is far less than that available in a mixture of polar/non-polar components, thereby exacerbating problems of adhesive splitting.
A need therefore exists for a pressure-sensitive adhesive system that bonds polar surfaces to non-polar surfaces with the strength exhibited by distinct layers of polar/non-polar adhesives. The present invention fulfills this need among others.